The present invention relates to gas lasers and in particular to gaseous ion lasers having an improved laser discharge tube.
There are presently several varieties of commercially-available gas ion lasers. One type has a discharge tube which uses a plurality of graphite discs within a gas-confining glass tube. See for example U.S. Pat. No. 3,619,810. In another type, thick-walled beryllia (BeO or beryllium oxide) segments are bonded together to make a discharge bore. See for example U.S. Pat. No. 3,760,213.
Another type of discharge tube is described in U.S. Pat. No. 3,501,714. In this patent a thin-walled, precision, ceramic tube is used. One of the suggested ceramic materials is alumina (Al.sub.2 O.sub.3 or aluminum oxide). Heat generated in the discharge is conducted out of the tube through a series of closely-spaced cylindrical sections which expand into contact with the ceramic tube when they are heated during operation of the tube.
This design has a number of advantages over the thick-walled BeO type discharge tube. In a BeO capillary tube with an inner bore diameter of about two millimeters and outer diameter of the order of one to one-and-a-half centimeters, the thermal heat flow through the beryllium oxide produces a tensile stress in the outer wall of the tube, which in a typical ion laser amounts to ten to fifteen percent of the breaking strength of the ceramic. Since alumina has about one seventh the thermal conductivity of beryllium oxide and since the stress is inversely proportional to thermal conductivity, the stress in an alumina capillary tube of the same dimensions could be sufficient to break the tube.
However, in a relatively thin walled tube (neglecting end effects) the circumferential and longitudinal stress tension in the outer layers is given by: (1/2) .DELTA.t.alpha..epsilon./(1-.nu.); where .DELTA.t is the temperature gradiant across the tube wall; .alpha. is the linear coefficient of expansion of the material, .epsilon. is Young's modulus for the material; and .nu. is Poisson's ratio. If the alumina tube is made on the order of 11/2 inches in diameter, the area available for heat flow substantially reduces .DELTA.t so that the stresses can be reduced essentially in the ratio of the outer diameter of the tube.
Also, the alumina tube can be made with a thinner wall. The thick walls in the beryllium oxide tube are for structural rigidity and also because a large surface area is needed for water cooling. In the aluminum oxide tube the large outer diameter provides both structural rigidity and the area available for water cooling. The calculated stresses in an aluminum oxide tube of 11/4 to 13/8 inches inside diameter and 11/2 to 15/8 inches outside diameter amount, at the same power flow through the tube, to ten to fifteen percent of the breaking strength of the aluminum oxide. Thus the thin-walled aluminum oxide tube is completely comparable to the beryllium oxide tube in its ability to conduct away the heat generated in the tube without producing a stress which is a large fraction of the tensile strength of the tube.
However, the laser tube described in U.S. Pat. No. 3,501,714 has many shortcomings both in design and fabrication. Many of these arise out of the use of a precision ceramic tube and high tolerance discs which are not permanently bonded to the wall of the tube.
U.S. Pat. No. 3,501,714 describes a gas laser tube that uses expansion of tightly toleranced discs, both in surface finish and in diameter to insure symmetric and uniform thermal coupling between the expanding discs and the ceramic tube wall. Besides making fabrication expensive and difficult, if these tolerances are not met the expansion of the inner disc combined with the asymmetric heat flow through the outer wall of the tube can lead to large tangential stresses exceeding the tensile strength of the ceramic tube. The tight tolerances that must be held limit the length of tube that may be machined. In the described laser, the discharge tube was less than 4" long even though the outer diameter of the tube was about 1.7".
Under symmetric radial heat flow conditions thermal stress in the ceramic tube wall is generated due to the fact that the inner wall is at a higher temperature than the outer wall. Hence it expands more producing a tangential tensile stress in the outer wall. Under the condition of symmetric radial heat flow the tangential tensile stress (and ceramic is weakest under tensile stress) in the outer wall is already approximately 20% of the tensile breaking stress of the ceramic. Even if extremely tight tolerances are maintained it is estimated that a condition of asymmetric heat flow could result and the tensile stresses in the outer ceramic tube would be a factor of 5 greater than the stresses that would be encountered in a situation of symmetric radial heat flow. This condition could easily lead to breakage of the tube.
Furthermore, the surface finish tolerances quoted (outside of the discs--16 RMS or better; surface finish on the inside wall of the alumina tube--32 RMS or better) limit the effective surface area available for heat flow through the tube walls.
The laser described in U.S. Pat. No. 3,501,714 depends upon the use of thick discs in order to maximize the surface area available for thermal transfer between the gas discharge and the outer ceramic tube envelope. This fact, and the poor thermal contact between the inner discs and the outer ceramic envelope contribute to a high impedance to gas flow within the laser in several ways. First, it is necessary to use a cylindrical channel through the thick discs for gas flow which creates a greater impedance to the gas flow. Secondly, the temperature of the discs is higher resulting in greater gas flow impedance.